Quenched and tempered low alloy steels are currently used in thick plate applications due to their good combination of hardness and toughness at a low cost. However, for weight sensitive applications, the high density of low alloy steels can be a significant concern. Fe-Mn-Al-C alloys are a promising low-density substitute for conventional steels. The addition of lighter elements like Al and C lower the density of the alloy. Precipitation strengthened austenitic Fe-Mn-Al-C alloy is studied in this work due to its comparable hardness and toughness to conventional steels. However, when welded, precipitation strengthened alloys can form low hardness regions in the heat affected zone (HAZ) due to precipitate dissolution or coarsening. Although, this can be corrected with a post weld heat treatment, it may not be feasible to heat treat after welding a large component or structure. Therefore, it is critical to understand the kinetics for precipitation, coarsening, and dissolution to propose the rejuvenation of hardness in the HAZ. Currently no matching filler metal exists for the Fe-Mn-Al-C alloys, therefore, potential filler metals with similar ageing kinetics as the base metal need to be investigated. The objective of this research is to develop a fundamental understanding of the effect of welding thermal cycles on the precipitation kinetics and the mechanical properties of both the fusion zone and the HAZ.
To simulate shop fabrication, repairs, and potential post weld heat treatments (PWHT), the HAZ mechanical properties were evaluated for aged and solution treated base metals in the as-welded and PWHT’ed conditions. A Gleeble thermo-mechanical simulator was used to reproduce different regions of the HAZ for mechanical testing and characterization at both low and high heat input weld conditions. The Sandia Smartweld package was used to calculate the thermal cycles associated with 1200°C, 1050°C, 880°C, 780°C, 600°C, and 400°C peak temperatures. The trends in hardness, tensile strength, and Charpy V-notch impact energy values were correlated with the microstructural observations for all the conditions. Digital image correlation (DIC) specimen were extracted from gas tungsten arc welds (GTAW) with the candidate filler metals. The DIC tests were conducted for single pass and multipass welds in both as welded and PWHT’ed conditions. The GTAW and HAZ specimens were characterized using LOM for grain size analysis, SEM and TEM for phase identification, and EBSD for orientation relationship between the grains.
Preliminary results from the Gleeble HAZ studies revealed the formation of low hardness regions in the HAZ with peak temperatures >600°C attributed primarily due to dissolution of kappa carbides. The slow ageing kinetics of kappa precipitates suggest that hardness rejuvenation from multipass welding may not be feasible and a PWHT is imperative to bring back the hardness in the HAZ. The effect of such PWHT on the mechanical properties in the HAZ will be discussed in detail and a correlation will be made with the observed microstructure. TEM imaging and diffraction experiments were conducted to characterize the kappa carbides (on the order of 5 to 20nm). A correlation will be made between the kappa precipitation (size, vol. fraction, location) and grain size on the deformation mechanism that will in-turn be correlated to the failure mechanisms during mechanical tests. Similar to HAZ, the effect of PWHT on the microstructure and mechanical properties in the fusion zone will be discussed. Candidate filler metals include the 316L, 17-4PH, and 13-8 alloys. Ageing experiments of single and multipass GTAW welds with the candidate filler metals revealed very similar ageing response as the Fe-Mn-Al-C base metal. The effect on tensile properties were evaluated using cross-weld DIC specimen to identify the failure location in each test conditions. The results from DIC experiments and corresponding microstructural observations will be discussed in detail. Thermo-calc DICTRA simulations were done to simulate the evolution of microstructure across the fusion line during ageing. The difference in chemical potential gradient across the fusion line led to the diffusion of Carbon during ageing thus forming new phases. The results from validation of DICTRA simulations through microstructural characterization will be presented in the talk.
Fe-Mn-Al-C alloys are a potential replacement for low alloy steels in weight sensitive structural applications providing good combination of hardness, toughness, and weight saving. Results from the microstructural characterization and their correlation with the observed trends in mechanical properties in HAZ and the fusion zone will be discussed in this talk.